Justin Bieber -- The Billionaire and a Boy Club

Justin Bieber and his abs hung out with billionaire Sir Philip Green aboard his yacht in Monaco this weekend ... appearing quite chummy.

It's a funny pairing -- Green made his fortune off clothing (Topshop) ... and JB rarely wears any.

While Justin's been showing off his street cred by hanging out with various rappers, there's still nothing cooler than a BILLION DOLLARS.